Osama bin Laden angrily vetoed a plot to fit rotating blades to a tractor and use it to "mow down the enemies of Allah", on the grounds that it would cause "indiscriminate slaughter", according to a US official familiar with material seized in the raid on the al-Qaida chief's hideout.

The hoard of documents and computer discs taken by US special forces consisted more of "strategic musings" than concrete terror plots, the official told ProPublica, the US non-profitmaking investigative news organisation.

Though a fierce advocate of mass-casualty attacks in the west – elsewhere, Bin Laden attempts to calculate the number of Americans who would have to be killed to force the US to withdraw from the Arab world – he appeared angry at a suggestion by the Yemen-based al-Qaida in the Arabian Peninsula that a tractor or farm vehicle could, suitably adapted, be used in an attack.

The official told ProPublica: "Bin Laden said this is something he did not endorse. He seems taken aback. He complains that this tactical proposal promotes indiscriminate slaughter. He says he rejects this and it is not something that reflects what al-Qaida does."

Bin Laden and his associates repeatedly admonished leaders of affiliated groups such as al-Qaida in the Arabian Peninsula or al-Qaida in Iraq for not paying sufficient attention to avoiding targets that could not be portrayed as "legitimate" to the leadership's target audience in the Middle East.

A series of letters and emissaries were sent to warn Abu Musab al-Zarqawi, the leader of al-Qaida in Iraq, to stop attacks on local Shia Muslims and to avoid civilian casualties. One senior al-Qaida figure close to Bin Laden told Zarqawi that he should be careful to avoid the fate of militants in Algeria in the 1990s who lost public support through indiscriminate and random violence.

Bin Laden also discusses which senior American political figures should be prioritised as targets. He dismissed an attack on Vice-President Joe Biden, saying he was of insufficient significance.

Other material seized shows how Bin Laden communicated with a small number of lieutenants mainly based in Pakistan, who then either executed his orders or tried to make sure all those in al-Qaida's disparate and scattered affiliates were aware of his directives. Missives were carried by couriers as Bin Laden had no internet or telephone access.

It was a courier who eventually led the Americans to the al-Qaida leader.